1. Field of the Invention
The present invention relates to methods for producing pyridine derivatives, and to intermediates in the synthesis thereof. The pyridine derivatives produced by the present invention are useful as intermediates in the synthesis of compounds having a pyridine skeleton in the molecule, such as furopyridine derivatives having antiviral activity (see WO 96/35678).
2. Description of the Related Art
Several methods have been disclosed in the past as methods for the synthesis of condensed pyridines such as furopyridines (see Heterocycles, 45(5) (1997), p. 975), but the only known methods for producing 5-substituted furo[2,3-c]pyridines which can be converted to the aforementioned furopyridine derivatives having antiviral activity are methods in which synthesis is achieved through multiple stages using 2-chloro-3-hydroxypyridine as the starting material (see WO 96/35678), and methods in which furfural oxime as the starting material is made into 5-substituted furo[2,3-c]pyridine-N-oxide as a result of ring formation by the Aza Diels-Alder reaction and subsequent dehydrogenation (see Tetrahedron Lett., 32, 3199(1991)).
Problems in the former of the aforementioned methods, however, are the large number of steps as well as the expensive starting materials and reaction agents, while the latter method needs expensive dehydrogenating agents such as 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), and needs the reduction of the pyridine oxide to pyridine, and hence both methods cannot be considered to be industrially useful.